Vehicle hydraulic brake systems comprise a brake fluid reservoir, which is usually seated on the master cylinder of the vehicle brake system and connected by means of two couplings in a fluid-conducting manner to the master cylinder. Hydraulic fluid, which is needed to balance the fluid content of the brake system, flows from said brake fluid reservoir into the brake system. Surplus brake fluid moreover flows from the brake system into the brake fluid reservoir. The brake fluid reservoir is accordingly used to provide a spare or buffer volume of brake fluid.
Because of the described flow of brake fluid into and out of the brake fluid reservoir, the pressure in the brake fluid reservoir may alter. Generally, and also according to DE 31 2 085 A1, a venting or pressure compensating device is therefore provided, usually on the filler neck of the brake fluid reservoir and on the lid cooperating therewith. Said venting and/or pressure compensating devices are designed for normal vehicle operation.
However, especially in the all-terrain vehicles recently being purchased in increasing numbers, the problem arises that brake fluid may escape from the brake fluid reservoir via the pressure compensating device when the vehicle is in a steeply inclined position. An escape of brake fluid is extremely undesirable because the brake fluid is highly inflammable and may therefore lead to a fire under certain circumstances, e.g. if escaping brake fluid drips onto hot engine parts, e.g. onto the exhaust manifold. Even if said risk did not exist, no brake fluid should escape from the brake fluid reservoir because brake fluid is toxic and harmful to the environment.